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United States Patent |
5,205,145
|
Ishino
,   et al.
|
April 27, 1993
|
Method of producing torque sensor shafts
Abstract
The invention relates to a method of producing magnetostriction type torque
sensor shafts adapted to detect a change in permeability when a torque is
applied. A plurality of shot peening operations are applied to the surface
of a torque sensor shaft in such a manner that the diameter of shot
particles to be used is decreased each time. This arrangement ensures that
the region in which the residual stress is at a maximum and approximately
constant lies over a wide range as seen in the direction of the depth from
the outermost surface of the shaft and extends from a deep area to an area
close to the surface of the shaft. As a result, the hysteresis and
nonlinearity of the shaft are improved and it becomes possible to use a
wide range of excitation frequencies.
Inventors:
|
Ishino; Renshiro (Hirakata, JP);
Shibata; Yoshio (Hirakata, JP)
|
Assignee:
|
Kubota Corporation (Osaka, JP)
|
Appl. No.:
|
876520 |
Filed:
|
April 30, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
72/53; 29/90.7; 310/26 |
Intern'l Class: |
B21D 031/06; H01L 041/12 |
Field of Search: |
72/53
73/660,DIG. 2,862.36
310/26
29/90.7
51/320
|
References Cited
U.S. Patent Documents
3073022 | Jan., 1963 | Bush et al. | 72/53.
|
4933580 | Jun., 1990 | Ishino et al. | 310/26.
|
Primary Examiner: Jones; David
Attorney, Agent or Firm: Farley; Joseph W.
Parent Case Text
This is a continuation-in-part of copending application Ser. No. 07/806,840
filed on Dec. 9, 1991 and now abandoned, which is a continuation of
application Ser. No. 07/586,423 filed on Sep. 21, 1990 and now abandoned.
Claims
What is claimed is:
1. A method of improving the magnetic performance characteristics of a
magnetostriction type torque sensor in respect of hysteresis, sensitivity
and non-linearity, said torque sensor having a torque sensor shaft,
comprising:
forming on the surface of said torque sensor shaft a magnetostrictive
portion comprised of a plurality of knurled grooves each having a radius
at its bottom;
applying to the surface of said torque sensor shaft including at least said
magnetostrictive portion a plurality of shot peening operations
successively using shot particles of different, decreasing diameters; and,
using for the second and any subsequent one of said shot peening operations
shot particles having a diameter smaller than two times said radius of
said knurled grooves.
2. The method set forth in claim 1, using two groups of shot particles of
different diameters.
3. The method set forth in claim 1, using three or more groups of shot
particles of different diameters.
4. The method set forth in claim 1 wherein the material of said torque
sensor shaft has a skin depth region in the outermost layer thereof, and
applying said shot peening operations so as to provide in said skin depth
region a compressive residual stress of not less than 20 kgf/mm.sup.2
within a range of not less than 0.1 mm depthwise from the surface of said
torque sensor shaft.
5. The method set forth in claim 1, further comprising subjecting said
torque sensor shaft to heat treatment subsequent to said forming operation
and prior to the application of said shot peening operations, and using in
said shot peening operations shot particles having a hardness greater than
the hardness of said magnetostrictive portion after said heat treatment
operation and prior to said shot peening operations.
6. The method set forth in claim 1 wherein said shot peening operations are
carried out so as to obtain a coverage of 98%.
Description
FIELD OF THE INVENTION
The present invention relates to a method of producing torque sensor shafts
and more particularly it relates to a method of producing magnetostriction
type torque sensor shafts adapted to detect changes in permeability
particularly during torque application.
BACKGROUND OF THE INVENTION
Magnetostriction type torque sensor shafts have been known, as one is shown
in Japanese Patent No. 169,326, wherein the surface of a sensor shaft
adapted to have torque transmitted thereto is formed with a magnetically
anisotropic section by forming spiral grooves therein by cutting or
rolling so as to detect changes in the permeability of the magnetic
anisotropic section, when torque is applied, to expess them in terms of
electrical quantities.
Hitherto, however, no torque sensor of such grooved type has been put in
actual use in the art. The reason for this is that a torque sensor shaft
constructed of a structural steel material through a mere process such
that the material is formed with spiral grooves and then subjected to
suitable heat treatment is liable to hysteresis, usually of the order of
about 2 to 20% FS, and could not be used as such in any practical
application. Recently, in order to put the basic principle of such torque
sensor shaft into practical use, a knurled type magnetostrictive torque
sensor has been proposed as disclosed in U.S. Pat. No. 4,933,580 of the
present inventors, wherein shot peening is applied to the grooved portion
to decrease hysteresis and improve sensitivity.
The achievement of hysteresis reduction through the process of peening the
grooved portion after heat treatment as described in U.S. Pat. No.
4,933,580 is explained by the fact that broadly peening has two kinds of
effect, mechanical and magnetic.
More specifically, the mechanical effect of shot peening includes the
effect of mending microcracks produced during the process of groove
forming, and the effect of improving the mechanical strength. The
improvement of the mechanical strength is brought about in the form of
hardened surface layer of the shaft and reduced crystal grain size of the
surface layer which result from the collision of small shot particles
blown. Such mechanical effect results in reduced hysteresis of the sensor.
The magnetic effect of shot peening includes improvement in the process of
surface magnetization and intensification of the magnetic anisotropy of
the sensor. The improvement of surface magnetization occurs as a result of
the fact that by virtue of shot peening the process of surface
magnetization changes from magnetization through domain wall displacement,
a process which tends to cause magnetic hysteresis, to revolving
magnetization, a process which is unlikely to cause magnetic hysteresis.
Such improvement in the process of magnetization results in reduced
hysteresis and improved sensor sensitivity. The intensification of the
magnetic anisotropy results from the fact that shot peening induces
development of residual stress on the shaft material as will be described
hereinafter. By virtue of the intensified magnetic anisotropy the
non-linearity of the sensor is corrected.
In the past, the effect of shot peening was well known in that the hardness
of the outermost surface layer of the shaft was increased and some
compressive residual stress was provided, which would result in improved
mechanical strength (fatigue strength). However, the past recognition in
the art that the effect of shot peening was limited to the improvement of
such mechanical strength involves the following inconsistency. While, as
is well known, shot peening brings about increased hardness of the surface
layer of the material and, in conjunction therewith, improved surface
layer strength of the material, it must be pointed out that the magnetic
hardness of the material is also increased and accordingly the retentivity
of the material becomes so large that the material can hardly be
magnetized further. As a matter of practice, therefore, mere improvement
in mechanical strength is generally likely to lead to decreased
sensitivity.
According to experiments conducted by the present inventors, wherein a
shaft material was hardened by carburizing, tempered and heat treated to
thereby increase the hardness of its surface layer, while for the purpose
of comparison a similar shaft material was hardened in a
carburization-prevented condition, and then tempered and heat treated,
that is, bright-handened, tempered and heat treated without so much
increase in the hardness of the shaft surface, the higher the shaft
hardness resulting from carburization, the lower was the sensitivity of
the torque sensor using the shaft.
As is apparent from this, the effect of shot peening presents some aspect
that cannot be explained only on the basis of increased mechanical
hardness and/or increased mechanical strength; and improvements in all
sensor characteristics, such as reduced hysteresis, reduced non-liniarity,
and improved sensitivity, can be obtained as an overall effect of shot
peening, or a combination of mechanical effect and magnetic effect as
above stated. In the earlier known shot peening technique as disclosed in,
for example, U.S. Pat. No. 3,073,022, the effect of shot peening for
mechanical strength improvement, as intended mainly for improvement of
fatigue strength, was only taught. In the prior art, the above cited U.S.
Pat. No. 4,933,580 was the first disclosure which referred to the above
stated magnetic effect.
More particularly, in the invention of a magnetostrictive torque sensor
described in U.S. Pat. No. 4,933,580, shot peening is applied to the
grooved portion of a sensor shaft and to areas therearound thereby to
mechanically, metallurgically and magnetically improve the outermost
surface layer of the grooved portion to reduce hysteresis and increase
sensitivity. That is, the following techniques are disclosed therein.
(1) Improvement of Mechanical and Metallurgical Strength of Grooves and
Areas therearound:
Application of shot peening to grooves and areas therearound martensitizes
the residual austenite in the outermost surface layer produced during
carburization to shafts to thereby increase hardness and it decreases
crystal grain size, whereby the strength of the outermost surface layer
through which magnetic flux passes is increased to a great extent.
As a result, when torque is applied, sufficient strength is provided to
resist the stress concentrated in the grooves. Even if a large stress is
applied, there is little possibility of producing a macroscopic mechanical
plastic deformation or a plastic deformation on the microscopic
crystalline level which causes the first mentioned plastic deformation or,
in other words, a magnetic plastic deformation. As a result, the
hysteresis characteristic is improved.
An auxiliary effect obtained is that the sensitivity is increased owing to
the nonmagnetic residual austenite being converted to ferromagnetic
martensite.
(2) Effect of Mending Microcracks in Grooves and Areas therearound:
Generally, microcracks are often produced in grooves and areas therearound
during machining, particularly rolling.
Such microcracks aggravate the hysteresis characteristic and lower the
sensitivity of sensors.
As is well known, shot peening has the effect of mending such microcracks
and hence it is useful for improving the hysteresis characteristic and
increasing the sensor sensitivity.
(3) Improvements in Magnetization of Outermost Surface Layers of Sensors:
Usually, magnetostrictive sensors are magnetized in a low magnetic field
having a magnetic intensity of tens of oersteds at 10 kHz to 100 kHz. In
most cases, the skin depth is about 0.1 mm immediately below the outermost
surface layer and the magnetization process is based mostly on the domain
wall displacement.
In this case, the presence of impurities and nonmagnetic inclusions in the
skin depth region forms a cause of magnetic hysteresis, and since shaft
materials in common use cannot avoid these impurities, it has been usual
that the hysteresis characteristic is bad.
On the other hand, application of shot peening results in forming
microscopic unevenness in the outermost surface layer of the grooves and
areas therearound which form the magnetically anisotropic section of the
sensor.
According to the teachings of physics of magnetism, formation of
microscopic pits on a metal surface by plastic deformation results in
formation of stable magnetic domains around the microscopic pits due to
annular residual stress, with the magnetization process in the annular
stable magnetic domains converting to magnetization rotation with less
magnetic hysteresis, thereby lowering the hysteresis characteristic of the
sensor.
With the above effects organically coupled together, application of shot
peening brings about a decrease in hysteresis and an increase in
sensitivity.
The action based on the effect (3) above is based on the action of residual
stress distribution applied to a region in the vicinity of the outermost
surface layer.
According to the known technique as described in the above cited U.S. Pat.
No. 4,933,580, when shot peening is applied to the surface of a sensor
shaft, only shot particles of uniform size are used. If the size of shot
particles is large, a stress distribution in which the compressive
residual stress is at a maximum and approximately constant is formed in a
deep region below the shaft surface in a wide range as seen in the
direction of the depth. Further, if the size of shot particles is small, a
peak value of compressive residual stress is obtained in a shallow region,
but in this case the region where the compressive residual stress is
approximately constant is narrow.
For this reason, in the case where the size of shot particles is small,
excitation conditions using high frequency ac currents are utilized to
ensure shallow penetration of magnetic flux; in this manner, optimum
excitation conditions are provided. However, since the region where the
compressive residual stress is approximately constant is narrow, it is
necessary that the range of utilizable excitation frequencies be from 50
kHz to 100 kHz, which are considerably high frquencies.
In the case where the size of shot particles is large, excitation
conditions using low frequencies (usually, about 10 kHz) to enable
magnetic flux to penetrate into depths are selected so that the depth of
penetration of magnetic flux is greater than when the size of shot
particles is small and so as to minimize the influence on the outermost
surface layer of the shaft where the changes in stress distribution is
large and where the compressive residual stress is small; in this manner,
hysteresis and sensitivity are improved. However, in this case, the region
of the shaft near its surface aggravates the sensor characteristics.
Therefore, when it is desired to obtain satisfactory hysteresis
characteristics, it is necessary to use a large excitation current.
In torque sensor shafts having such conventional shot peening methods
applied thereto, the range in which the compressive residual stress is
approximately constant does not necessarily have a sufficient expanse, so
that there is a problem that the range of usable excitation frequencies is
narrow.
DISCLOSURE OF THE INVENTION
The present invention is intended to solve such problems and its object is
to enlarge the range in which the compressive residual stress is
approximately constant from the outermost surface layer toward the shaft
center to thereby enlarge the optimum excitation frequency range.
To achieve this object, in accordance with the invention, a method of
producing torque sensor shafts of magnetostrictive type adapted to detect
changes in permeability when torque is applied, is characterized in that a
plurality of shot peening operations are applied to the surface of a
torque sensor shaft material including at least a magnetostrictive portion
by successively using shot particles of different diameters, from larger
to smaller, whereby a torque sensor shaft having improved mechanical
characteristics and, in particular, improved magnetic characteristics is
obtained which is suitable for use in making a magnetostriction type
torque sensor having improved performance characteristics in respect of
hysteresis, sensitivity, and non-lininearity in particular, based on
stress and magnetic characteristics of the sensor, when the torque sensor
shaft is used in making the torque sensor.
With this scheme, first with shot particles of large diameter, a
distribution in which the compressive residual stress is at a maximum and
constant is obtained in a deep region below the shaft surface.
Subsequently, by successively applying shot peening operations using shot
particles of successively smaller diameters, the region in which the
compressive residual stress is at a maximum and approximately constant is
progressively enlarged toward a shallower region below the outermost
surface layer of the shaft. Therefore, finally, the region in which the
compressive residual stress is at a maximum and approximately constant
lies over a wide range as seen in the direction of the depth. As a result,
a reduction in hysteresis and an increase in sensitivity are attained and,
as shown in the following embodiments, the nonlinearity can be improved
and it becomes possible to use a wide range of excitation conditions.
The relation of the present invention to above cited U.S. Pat. No.
4,933,580 and also to above cited U.S. Pat. No. 3,073,022 will be
explained below. The present invention is an improvement on the invention
of U.S. Pat. No. 4,933,580. According to the invention of U.S. Pat. No.
4,933,580, the hysteresis level which was as high as 2 to 20% FS in the
case of no shot peening being effected can be improved to a level of not
more than 1% FS. In contrast to this, the present invention provides for
further improvement in hysteresis and, in addition, for good improvement
in sensitivity and nonlinearity; thus, sensor characteristics can be
remarkably improved.
In the description to follow, the process of shot peening in which one kind
of shot particles with single particle size is used is referred to as
"single peening", and the process of shot peening in which two kinds of
shot particles different in particle size is referred to as "double
peening".
Referring to the aspect of mechanical strength improvement, it is already
known that improvement in mechanical strength is obtained in the case
where single peening is effected. It is also known from the teaching of
U.S. Pat. No. 4,933,580 that shot peening results in a decrease in the
hysteresis of a torque sensor. Again, it is already known from the
teaching of U.S. Pat. No. 3,073,022 that where double peening is effected,
greater improvement in mechanical strength can be obtained as compared
with the case of single peening being effected. It may possibly be
inferred from a combination of teachings of U.S. Pat. No. 4,933,580 and
U.S. Pat. No. 3,073,022 that double peening would bring about greater
improvement in hystersis characteristic than single peening, but nothing
can be inferred for the aspect of sensor sensitivity and nonlinearity
improvement.
Referring to the aspect of magnetic characteristic improvement, it is
already known that where single peening is applied, the hysteresis of
torque sensors is reduced and sensor sensitivity is improved. In U.S. Pat.
No. 3,073,022, however, no teaching is given with regard to improvement in
magnetic characteristic by double peening. It has for the first time been
disclosed by the present invention that double peening can bring about
further improvement in magnetic characteristic and can amazingly improve
sensor characteristics. Nonlinearity in particular cannot be improved
unless improvement in magnetic characteristic is achieved.
The effect of shot peening for magnetic characteristic improvement will be
explained in detail hereinbelow.
The effect of improved magnetization for hysteresis reduction is first
described in detail, although it has already been briefly referred to.
When shot peening is applied, an annular compressive residual stress will
develop around shot depressions. This annular region of compressive
residual stress acts as a magnetic domain, and the process of its
magnetization is such that a rotating magnetization process is predominant
wherein magnetic rotation occurs in the direction of an acting stress when
the stress is applied. Generally, the process of domain wall displacement
is non-reversible and is likely to cause magnetic hysteresis, whereas the
process of rotating magnetization is reversible and is unlikely to cause
magnetic hysteresis. As a result of shot peening, the magnetizing process
in which a non-reversible process of domain wall displacement has been
predominant changes into a reversible process of rotating magnetization,
and this leads to improvement in the hysteresis characteristic.
Nextly, the effect of sensitivity improvement through improvement of the
magnetizing process will be explained. The entire process of magnetization
(M) consists of the magnetizing process of domain wall displacement (MW)
and the magnetizing process of rotating magnetization (MR). The
magnetizing process of domain wall displacement (MW) is divided into a
90.degree. domain wall displacement process (MW 90) and a 180.degree.
domain wall displacement process (MW 180). This may be expressed by the
following equations:
M=MW+MR
M=MW90+MR180+MR
The magnitude of the entire magnetizing process (M) is proportional to the
permeability or a voltage detected when no torque is applied. It is the
90.degree. domain wall displacement process (MW 90) and rotating
magnetization process (MR) that relates to magnetostriction
characteristics which influence the sensitivity characteristic. In the
present invention, a greater part of the magnetizing process of domain
wall displacement (MW 90, MW 180) changes into the rotating magnetization
process (MR) because of the annular compressive residual stress due to
shot peening and, therefore, the proportion of the magnetizing process
which influences the sensitivity characteristic is increased, so that
sensitivity improvement can be obtained.
Further, according to the invention, improvement in nonlinearity is
obtained through the improvement in the magnetizing process, which will be
explained on the basis of the following description of the embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a distribution of residual stress in the
interior of a torque sensor shaft according to an embodiment of the
invention;
FIG. 2 is a diagram showing frequency characteristics versus hysteresis and
nonlinearity for a torque sensor using a torque sensor shaft representing
an embodiment of the invention;
FIG. 3 is a view showing a knurled portion of a torque sensor of knurled
type;
FIGS. 4 to 9 are sectional views showing modified forms of the knurled
portion; and
FIG. 10 is a diagram showing the relationship between shot peening coverage
and sensor hysteresis.
DESCRIPTION OF EMBODIMENTS
FIG. 1 shows a distribution of compressive residual stress in the interior
of a shaft when the surface of the material (structural steel SNCM 815,
specified in JIS (Japanese Industrial Standard)) of the shaft is subjected
to shot peening first with shot particles of large diameter (0.6 mm) and
then with shot particles of small diameter (44 .mu.m). The vertical axis
represents the magnitude of the compressive residual stress and the
horizontal axis represents the depth measured from the shaft surface. In
the diagram, the dash-dot line indicates a compressive residual
distribution obtained by effecting shot peening with shot particles of
large diameter (0.6 mm) alone and the broken line indicates a compressive
residual stress distribution obtained by effecting shot peening with shot
particles of small diameter (44 .mu.m) alone. The solid line indicates a
stress distribution obtained by effecting shot peening first with shot
particles of large diameter (0.6 mm) and then with shot particles of small
diameter (44 .mu.m).
As previously described, in the case where shot peening is effected with
shot particles of large diameter (0.6 mm) alone, an area where the
compressive residual stress is at a maximum and approximately constant
appears in a deep region from the shaft surface in a wide range (from 0.05
mm to about 0.15 mm) as seen in the direction of the depth. In the case
where shot peening is effected with shot particles of small diameter (44
.mu.nm), an area where the compressive residual stress is at a maximum and
approximately constant appears in a shallow region from the shaft surface
in a narrow range (from about 0 to 0.05 mm).
In the case where shot peening is effected first with shot particles of
large diameter (0.6 mm) and then with shot particles of small diameter (44
.mu.m), the range in which the compressive residual stress is at a maximum
and approximately constant is wide (from about 0 to 0.15 mm in the
direction of depth), extending close to the shaft surface. Therefore, as
compared with the case of using shot particles of one fixed diameter
alone, it becomes possible to use a wide range of excitation conditions to
enlarge the range of use of torque sensors.
FIG. 2 shows an example of frequency characteristic versus hysteresis and
nonlinearity for a torque sensor shaft produced by the method of the
present invention. The shaft material used was SNCM 815 steel specified in
JIS. The shaft was subjected to shot peening first with steel particles of
0.6 mm in diameter at a shot pressure of 7 kgf/cm.sup.2 and a coverage of
not less than 70% and then with steel particles of 44 .mu.m in diameter at
a shot pressure of 5 kgf/cm.sup.2 and a coverage of not less than 70%. The
sensor characteristic was measured by constant voltage drive with an
effective excitation current of 35.5 mA while changing the frequency. In
the figure, the lower group of curves show the hysteresis and nonlinearity
measured. The upper group of curves are for a comparative example, showing
the result obtained by a measurement under the same conditions for the
same shaft material when shot peening was effected only with steel
particles of 0.6 mm in diameter at a shot pressure of 7 kgf/cm.sup. 2 and
a coverage of not less than 70%.
As is clear from the figure, according to the method of the present
invention, as compared with the case of using only one kind of particles,
not only hysteresis but also nonlinearity is improved.
As for the magnitude of the compressive residual stress in the outermost
surface layer due to shot peening, in the case of shot peening with steel
shots of 0.6 mm in diameter, as shown in FIG. 1, a compressive residual
stress of about 50 kgf/mm.sup.2 appears both axially and
circumferentially, while application of shot peening first with shots of
0.6 mm in diameter and then with shots of 44 .mu.m in diameter results in
a compressive residual stress of about 70 kgf/mm.sup.2 appearing both
axially and circumferentially, achieving improvements in hysteresis,
nonlinearity and sensitivity.
Experiments of shot peening were conducted by changing shot peening
conditions. As a result, it was found that if the compressive residual
stress in the outermost surface layer was not less than about 20
kgf/mm.sup.2 and a region where the compressive residual stress was not
less than about 20 kgf/mm.sup.2, extends from the outermost surface layer
to an area not less than 0.1 mm deep, this was very effective for
improving hysteresis, nonlinearity and sensitivity.
What should be noted here is that while U.S. Pat. No. 4,933,580 discloses
that shot peening is useful for greatly decreasing hysteresis and
increasing sensitivity, the present invention has proved that shot peening
is also useful for improving nonlinearity. In other words, special mention
should be made of the fact that shot peening greatly improves all of the
fundamental performance characteristics required of a sensor, i.e.,
hysteresis, nonlinearity and sensitivity; thus, the utility value of shot
peening is very high.
Further, as is clear from FIG. 2, according to the present invention, as
compared with the case of effecting shot peening with steel particles of
0.6 mm in diameter alone, both hysteresis and nonlinearlity are improved
over the entire range of excitation frequency used in measurement. The
reason is that shot peening first with steel particles of 0.6 mm in
diameter and then with steel particles of 44 .mu.m in diameter ensures
that the residual stress distribution in the vicinity of the outermost
surface layer where a maximum stress occurs when stress is measured by a
torque sensor is a uniform distribution in which when excitation is
effected using a wide excitation frequency range from 10 kHz to 100 kHz,
the residual stress is at a maximum and uniform over the entire skin depth
range through which the magnetic flux passes.
The reason why nonlinearity is improved as described above is that the
magnetic anisotropy of grooves is heightened by shot peening.
According to a recent study of the present inventors, application of shot
peening to the grooved portions which are magnetically anisotropic
portions results in generation of residual stress in the magnetically
anisotropic portions. As may be understood from the knurling configuration
shown in FIG. 3, in the case where the dimension in the direction of the
grooves is longer than that in a direction orthogonal thereto (i.e., in
the direction of the width), in other words, in the case where elongate
grooves are formed, compressive residual stress due to shot peening may be
expressed by the following relation: (compressive residual stress in the
direction of the grooves)<(compressive residual stress in a direction
orthogonal to the direction of the grooves) (see Symposium Material
entitled "Residual Stresses and Shot Peening", Jan. 28, 1990, p. 19, Isuke
Iida, Professor at Meiji University). Thus, the direction becomes the easy
direction of magnetization. Primarily, the grooves which are of an
elongate configuration have a geometrical magnetic anisotropy with an
easy-to-magnetize axis. Yet, a further magnetic anisotropy due to residual
stress is provided by shot peening, and this results in improved
nonlinearity and further hysteresis reduction.
Further, according to the present invention, since shot peening is effected
first with shot particles of large diameter and then with shot particles
of small diameter, microcracks often produced by shot particles of large
diameter are mended by shot particles of small diameter. As a result, the
surface quality of the torque sensor shaft is improved and the strength of
the shaft is increased; this fact also contributes to improvement in
hysteresis and nonlinearity characteristics.
Nextly, the relationship between shot-particle size and groove bottom
radius will be explained. In the knurled groove portion shown in FIG. 3,
the groove bottom is subject to large stress concentration and, therefore,
should preferably have a radius of not less than 0.2 mm. Further, in order
to provide improved mechanical strength and improved magnetic
characteristic through shot peening, it is necessary that shot particles
should uniformly impinge upon the entirety of the knurled portion, and the
coverage should preferably be not less than 98%. Generally, in the art of
shot peening, a 98%, coverage is called "full coverage".
The knurled portion shown in FIG. 3 has a groove pitch P of 2 mm, a groove
length L of 15 mm, a groove bottom radius R of 0.4 mm, and a groove height
D of 1 mm. The knurled portion was subjected to shot peening with two
kinds of shot particles different in particle diameter. First shot
particles had a nominal particle diameter of 0.6 mm (SAE S170) (JIS
S-S160), a median particle diameter of 0.6 mm, and a hardness of Hv=700.
Second shot particles had a nominal particle diameter of 44 .mu.m, a
median distribution of 44 .mu.m, a distribution of not more than 90 .mu.m,
and a hardness of Hv=700. Shot peening conditions were: arc height after
first shot, 0.30 mmA; arc height after second shot, 0.33 mmA; coverage
after first shot, 200%; and coverage after second shot, 600%.
In this way, in order to achieve good shot peening effect, it is important
that the nominal particle diameter for second and subsequent, if any,
shots in particular be smaller than two times the radius R of the rounded
groove bottom. In other words, it is essential that the following relation
should hold:
.phi.s(mm)<2R(mm)
This relation should hold constant even when the sectional configuration of
grooves of the knurled portion varies in different ways as shown in FIGS.
4 to 9. This relation may hold true with respect to particle diameters of
not only second and subsequent shots but also of first shot.
FIG. 10 shows the results of a study on the relationship between coverage
and hysteresis. Improvement in sensor characteristics can be obtained when
the coverage is 70% and above, but preferably the coverage should be 98%
and above.
Nextly, mention is made of heat treatments. In order to obtain improved
fatigue strength of sensor shafts, it is effective to subject the sensor
shaft first to heat treatments in general practice, such as hardening by
carburization, induction hardening, carbontriding, and nitride heat
treatment, and subsequently to subject it to shot peening first with shots
of large diameter particles and then with shots of small diameter
particles.
Finally, the relationship between the hardness of a shaft material heat
treated but prior to shot peening and the hardness of shot particles will
be explained. In order that shot peening may result in satisfactory
compression hardening and good improvement in magnetic characteristics of
the surface layer of the magnetically anisotropic section of the sensor
shaft, it is desirable that the hardness Hvs of shot particles be greater
than the surface hardness Hvk of the magnetically anisotropic section. In
other words, it is desirable that the following relation should hold:
Hvs>Hvk
The foregoing example refers to the case of using two groups of shot
particles having large and small diameters respectively. However, three or
more groups of shot particles differing in diameter may be used so long as
a plurality of shot peening operations are effected using shot particles
with successively decreasing diameters. If, however, the order is reversed
to use shot particles of small diameter first and then shot particles of
larger diameter, the effects brought about by shot particles of small
diameter are killed by shot particles of larger diameter, so that the
intended frequency characteristics cannot be attained. Similarly, if a
mixture of shot particles of large and small diameters is used for a
single shot peening operation, neither hysteresis nor nonlinearity is
improved.
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